Electronic devices such as computers often include a real time clock circuit to keep track of a time and date. The time and date can then be used in a variety of important functions, including displaying the time and date to a user, time stamping operations performed by the electronic devices, etc. For example, when a data file is created or modified in a computer, a time and date stamp on the data file may be updated to identify when the change took place. When data is gathered by an electronic data collection system, a time and date stamp may be stored along with the data. Real time clock circuits may be used in a wide range of electronic devices for many different applications.
The time and date in a real time clock may often be manually or automatically set by an external source, such as a manual data entry by a user or by a connection to a time server on the Internet. Thereafter, the real time clock is generally expected to correctly keep track of the time and date, even if the electronic device is turned off and power is removed, so that the time and date are correct when the device is next powered on. A backup battery may therefore be connected to the real time clock to power the real time clock even when the host device containing the real time clock is turned off and power is removed. The life of a backup battery for a real time clock is particularly important because they are often relatively inaccessible in the host device and may not be user replaceable. For example, the backup battery for a real time clock in a computer is generally expected to last throughout the multi-year life of the computer. This requirement may be addressed to some extent by including a high capacity backup battery, although this increases the cost of the host device and may be less effective than desired if the real time clock uses power inefficiently while tracking the time and date when the host device is powered down.